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How a switch handles a broadcast.

In the "Cisco LAN Switching" by Clark and Hamilton you can read:

"Recall that SC0 is the management interface used in Catalyst switches such as the 4000s, 5000s and 6000s.

This is where the management IP address is assigned to a Catalyst Supervisor.

Because the CPU processes all broadcast packets (and some multicast packets) received on this interface, it is important to not overwhelm the CPU."

"Never mix end-user traffic with control and management traffic.

When implementing this principle, you must generally choose one of two designs:

- Use VLAN 1 for all control and management traffic while placing end-user traffic in other VLANs (VLANs 2–1000).

- Use VLAN 1 for control traffic, another VLAN (such as VLAN 2) for management traffic, and the remaining VLAN for end-user traffic (such as VLAN 3–1000)."

It seems that a broadcast is handled in different ways if it belongs to different VLANs. This principle is reasonable. But I have seen different resources where an output line of the command show mac-address-table is:

All    ffff.ffff.ffff    STATIC      CPU

It seems that a broadcast is handled by the CPU for all VLANs and this is not reasonable.

I do not know exactly which models those output lines belong to, but I think that principles are not changed since 4000s, 5000s and 6000s. May be the output line remember the switch that broadcasts are not handled as unicasts, and after that only broadcasts

in particular VLANs are sent to the CPU. May be that only because broadcasts must be flooded, this process requests a CPU intervention but it is not reasonable. It should be reasonable that after the decision of VLAN management traffic, only broadcasts in VLAN 1 and in VLAN management if different are sent to the CPU, broadcasts in VLANs user traffic should be switched by the switch fabric. What do you think about?



Re: How a switch handles a broadcast.

When one transmitter needs to reach all of the receivers in the network, it sends a broadcast. Broadcast frames are received by every host on the segment. The broadcast domain at Layer 2 is referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive broadcast frames from a host to all other machines on the LAN.

A switch is a Layer 2 device. When a switch receives a broadcast, it forwards it to each port on the switch except the incoming port. Each attached device must process the broadcast frame.

When two switches are connected, the size of the broadcast domain is increased.

The overall result is a reduction in available bandwidth as all devices in the broadcast domain must receive and process the broadcast frame.




Re: How a switch handles a broadcast.

Hello Naidu and thanks for your answer. My question is not about switch forwarding logic.

My question is about switch software and hardware architectures.

Is it true that not all VLANs broadcast are processed by the CPU?

Is it true that a broadcast which belongs to a user VLAN does not need to be processed by the CPU?

Is it true that only a broadcast which belongs to VLAN 1 and management VLAN needs to be processed by the CPU?

If these are true, why the output line of the command show mac-address-table shows All     FFFF.FFFF.FFFF     Static     Cpu ?

It would seem that all the broadcasts are sent to the cpu, so it would not depend on the VLAN.

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